Method for locking an artificial facet between two vertebral bodies

ABSTRACT

A bone screw ( 1 ) is used for locking an articular facet between the superior and inferior articular processes of two vertebral bodies and is provided with a threaded shaft ( 2 ), a screw head ( 3 ) and a central axis ( 4 ). The screw head ( 3 ) is further provided with a number of grooves ( 5 ) on the periphery of the screw head ( 3 ) which run essentially parallel to said central axis ( 4 ).

This is a continuation of International Patent Application No.PCT/CH2002/000608, filed Nov. 13, 2002, the entire contents of which areincorporated herein by reference thereto.

This invention concerns a bone screw, in particular for locking anarticular facet between the superior and inferior articular processes oftwo vertebral bodies.

To achieve a circumferential arthrodesis (fusion) the anterior and theposterior columns must be treated. The goal of the treatment is therestoration of the lordotic curve and the anatomically correct discspace. Anteriorly, i.e. in the intervertebral space, implants like cagesare inserted after disc removal. The posterior vertebral column, wherethe articular facet is located, should be locked as well.State-of-the-art techniques consider translaminar screws or transpedicalinstrumentation which, however, are not satisfactory.

Translaminar screws have certain disadvantages, like

a) the screw insertion point is difficult to localize; the localizationis done under full view, i.e. a separate posterior incision is performedmedially whereas muscles must be separated from spinal and laminarprocesses. Most morbidity results from medial incision;

b) the surgeon's view is two-dimensional due to the small incision whichmay result in interference of the second screw with the first one; and

c) aiming devices or navigation tools did not improve insertiontechnique or precision of screw placement.

From US-A 2001/007074 A1 STROBEL a bone screw is known.

From U.S. Pat. No. 4,754,749 TSOU another bone screw is known which hastwo closed canals in the screw head at an angle with the screw axis. Aguide pin is insertable in one of these dosed canals, so that uponinsertion of the guide pin it projects radially over the shaft andaxially over the head of the screw which renders it complicated andrather unstable.

The invention as claimed aims at solving the above described problems.

The present invention provides a bone screw and a method for locking anarticular facet of a vertebral body.

The advantages of said method and the use of said bone screw are thefollowing:

-   -   percutaneous approach which results in reduced morbidity;    -   low implant cost due to simple design, in particular in        comparison to transpedicular fixation;    -   lower risk with regard to screw positioning compared to        translaminar screws; and    -   easy removal of the screws.

The interference screw according to the invention allows a new surgicaltechnique to lock the articular facets of vertebral bodies.

The natural functional spine unit (FSU) contains two articular facets.The function of the screw according to the invention is the interferencein the sense of obstruction or fixation of said articular facets. Sincethe core diameter of the screw is significantly larger than the gap inthe articular facet, the device is hindering the natural articulation.The function of the screw thread is the insertion by rotation. Afterinsertion the screw thread protects the screw from axial migration andthe anti-rotation device protects the screw from migration by rotation.

According to the new surgical method the locking screws are insertedthrough two percutaneous approaches in the trajectories parallel to thearticular surface of the articular facet. Aiming wires guarantee thecorrect positioning. An anti-rotation element keeps the bone screw inposition and hinders the screws from turning out if micro-motion isapplied to the screws. This technique is applicable if the anteriorvertebral column (i.e. the intervertebral space of the related segment)is stabilized with a spacer such as an intervertebral cage.

According to a special embodiment the grooves on the periphery of thescrew head are running essentially parallel to the central axis. Thisshall be interpreted in such a way that minor angulation with regard tothe central axis either towards radially the latter or tangentially toit would still be functional. Such an angulation in a radial plane andmeasured relative to the central axis might be in the order of up to60°, but preferably lees than 20°. The possible angulation in atangential plane and measured relative to the central axis might be inthe order of maximum 20°, preferably less than 10°.

In a particular embodiment the bone screw is provided with at least onepair of diametrally opposed grooves on the periphery of said screw headwhich enhances stability of the implant.

The screw head may be provided with a central cavity coaxially arrangedwith respect to said central axis, e.g. with a polygonal profile,preferably a hexagonal profile for receiving a screw-driver having acorresponding profile.

The grooves on the periphery of the screw head may be juxtaposed to thepolygonal planes of said central cavity. By this measure a highermechanical strength can be achieved.

In a further embodiment an anti-rotation element is insertable in saidgroove or said pair of grooves on the periphery of said screw head,whereby said anti-rotation element in its inserted position projectsradially out of the periphery of said screw head. The anti-rotationelement is preferably a U-shaped staple with two legs and a centralportion bridging said two legs and designed for insertion into saidgroove or said pair of grooves of said screw head. The anti-rotationelement has preferably a diameter which is larger than said screw head.The anti-rotation element in form of a U-shaped staple may be providedwith a guiding element attached to said central portion and runningessentially parallel to said legs. The guiding element may be in theform of a plate, a circular cylinder or a prism designed for insertioninto said central cavity of said screw head. The cylindrical shape ofthe guiding element has the advantage of a more accurate gliding.

The central portion of the anti-rotation element may be provided with atleast one perforation for removal of the screw.

The threaded shaft of the bone screw has preferably a thread with a highangle of pressure, e.g. in the range of 4° to 70°. The flank of saidthread can be symmetrically or asymmetrically oriented. Theasymmetrically oriented thread is compressing particularly cancellousbone. This increases initial fixation stability.

The bone screw may be self-tapping, preferably by means of a cuttingedge.

The core of the screw shaft may be either cylindrical or tapering awayfrom the screw head.

In the case of cylindrical core of the screw shaft various advantagescan be achieved, namely:

-   -   a continuous bending strength along the screw shaft;    -   the application of a constant insertion torque while turning the        screw in the articular facet; and    -   due to the constant shaft diameter the screw does not become        loose if the device is turned back slightly after insertion.    -   This could appear if the surgeon brings the grooves for the        anti-rotation element in congruent direction like the articular        facet gap.

In the case of a conical shape of the core the shaft is compressing thesurrounding bone. This increases the initial stability of the implant.

The envelope of the threaded shaft may be cylindrical allowing also aconstant insertion torque. However, the envelope of the threaded shaftpreferably tapers away from the screw head so that the purchase of thethread in the bone is increasing by turning the screw in.

The bone screw may be self-drilling, preferably by means of a chuckinggroove.

The new method for locking an articular facet between the superior andinferior articular processes of two vertebral bodies consists in theinsertion of the threaded shaft of a bone screw in the gap of saidarticular facet. To that purpose the bone screw is preferably cannulatedand insertion is performed by means of an aiming wire. The bone screwhas preferably a screw head with a larger diameter than said threadedshaft and said threaded shaft is inserted in said gap of said articularfacet until said screw head touches the bone. Upon insertion of saidbone screw an anti-rotation element may be applied to said screw headsuch that rotation of said bone screw is prevented.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming partof this disclosure. For the better understanding of the invention, itsoperating advantages and specific objects attained by its use, referenceshould be had to the accompanying drawings, examples and descriptivematter in which are illustrated and described preferred embodiments ofthe invention.

In the Drawings:

FIG. 1 is a perspective view of the bone screw according to theinvention together with an anti-rotation element to be used with thescrew;

FIG. 2 a is a longitudinal section through the central axis of the screwand the anti-rotation element according to FIG. 1;

FIG. 2 b is a top view of the bone screw according to FIG. 1 with theanti-rotation element inserted into the screw head;

FIG. 3 is a perspective view of a trocar for bringing an aiming wireinto the gap between the articular facet;

FIG. 4. is a perspective view of a drill bit, the aiming wires beingtemporarily fixed in the gaps of the articular facets;

FIG. 5 is a perspective view of the bone screw being inserted by meansof a screw-driver into the gap of the articular facet by using thetemporarily fixed guiding wire;

FIG. 6 is a perspective view of the anti-rotation element being put overthe screw head into its grooves closest to the joint gap; and

FIG. 7 is a perspective view of the inserted bone screw to which theanti-rotation element has been attached.

The bone screw 1 as represented in FIGS. 1, 2 a and 2 b is used inparticular for locking an articular facet between the superior andinferior articular processes of two vertebral bodies. It has a threadedshaft 2, a screw head 3 and a central axis 4. The screw head 3 isprovided with six grooves 5 regularly disposed on the periphery of thescrew head 3 running essentially parallel to the central axis 4. Thescrew head 3 is further provided with a central cavity 6 coaxiallyarranged with respect to the central axis 4 and having a hexagonalshape.

The anti-rotation element 10 is provided with a U-shaped staple havingtwo legs 11 and a central portion 13 bridging said two legs 11. TheU-shaped staple is provided with a guiding element 12—having the shapeof a circular cylinder—attached to the central portion 13 and runningessentially parallel to the legs 11. As can be seen in FIGS. 1 and 2 athe anti-rotation element 10 may be connected to the bone screw 1 bymoving it along the central axis 4 whereby its central portion 13 entersthe central cavity 6 of the screw head 3 and the two legs 11 areinserted into one of the three pairs of grooves 5 of the screw head 3 asshown in FIG. 2 b.

The free ends of the two legs 11 are provided with an protrusion 15oriented radially inwards to the central axis 4 so that when the legs 11are gliding along the grooves 5 the protrusions 15 will click under thelower edge 16 of the screw head 3 thereby securing the anti-rotationelement 10 against withdrawal in the opposite axial direction.

The central portion 13 of the U-shaped staple is further provided withat a perforation 14 facilitating removal of the bone screw 1.

Useful materials for the bone screw 1 as well as for the anti-rotationelement 10 are titanium, titanium alloys or fiber-reinforced plasticmaterials. They may be coated with ceramic.

A detailed method of operation follows for the better under-standing ofthe invention:

-   1. Both positions in extension of the articulating planes of the    articular facet concerned are identified and marked accordingly on    the skin. To this purpose an image intensifier is used to control    position and direction.-   2. Bilateral skin incisions are performed in the direction of the    articular facets.-   3. Depending on the surgeon's preference, a trocar 17 or similar    instrument is used to bring an aiming wire 18 into the gap 19    between the articular facets (FIG. 3). Positional control is    indicated using an image intensifier.-   4. As shown in FIG. 4 the aiming wires 18 are temporarily fixed in    the gaps 19 of the articular facets by means of the drill bit 20    (“screw head reamer” or “counter sink”).-   5. As shown in FIG. 5 a cannulated and self-tapping bone screw 1 is    inserted by means of a screw-driver 21 into the gap 19 by using the    temporarily fixed guiding wires 18 until the screw heads 3 of the    bone screws 1 are touching the bone.-   6. As shown in FIG. 6 an anti-rotations element 10 is put over the    screw head 3 in the grooves 5 (notches) closest to the joint gap 19.    Eventually the bone screw 1 must be turned back by some degrees in    order to match the grooves 5 (notches) in the screw head 3 with the    joint gap 19 (FIG. 7).-   7. All instruments are removed.-   8. The placement of the bone screws 1 is verified by using an image    identifier and the wound is closed.

The method of operation as described can be performed on one side of thevertebral column only but is preferably performed simultaneously on theright and left side, as shown in FIGS. 3 to 7, which has biomechanicaladvantages.

1. A method of inserting an intra-facet bone screw into a facet jointbetween a superior articular surface of an inferior vertebral body andan inferior articular surface of a superior vertebral body, said methodcomprising: inserting a threaded shaft of a bone screw into the facetjoint between the superior articular surface of the inferior vertebralbody and the inferior articular surface of the superior vertebra so thatthe threaded shaft of the bone screw partially engages the superior andinferior articular surfaces, a longitudinal axis of the bone screw beingsubstantially aligned with and located between the superior articularsurface of the inferior vertebral body and the inferior articularsurface of the superior vertebral body; and attaching an anti-rotationelement on said bone screw such that a portion of said anti-rotationelement projects radially outward from the periphery of said bone screwto prevent rotation of said bone screw.
 2. The method of claim 1wherein: said bone screw is cannulated; and said inserting comprisesinserting said threaded shaft of said bone screw into the facet jointusing an aiming wire.
 3. The method of claim 1 wherein: said bone screwhas a screw head with a larger diameter than said threaded shaft; andsaid inserting comprises inserting said threaded shaft of said bonescrew into the facet joint until said screw head touches bone.
 4. Themethod of claim 1 wherein: said bone screw has a screw head having atleast one groove; and said attaching comprises attaching theanti-rotation element on said bone screw such that a portion of saidanti-rotation element is inserted in said groove and projects radiallyoutward from the periphery of said bone screw to prevent rotation ofsaid bone screw.
 5. The method of claim 1 wherein said inserting andsaid attaching are performed on right and left sides of a vertebralcolumn in a single surgical procedure.
 6. A method of inserting firstand second intra-facet bone screws into first and second articular facetjoint gaps between superior and inferior articular processes of twovertebral bodies of a patient's spine, the method comprising the stepsof: a) inserting a portion of a first aiming wire into the firstarticular facet joint gap and a portion of a second aiming wire into thesecond articular facet joint gap; b) simultaneously inserting the firstintra-facet bone screw into the first facet joint gap and the secondintra-facet bone screw into the second facet joint gap, the first bonescrew being inserted into the first facet joint gap between the superiorarticular process of an inferior vertebra and the inferior articularprocess of a superior vertebra such that an externally threaded shaft ofthe first bone screw partially engages the superior articular process ofthe inferior vertebra and the inferior articular process of the superiorvertebra, a longitudinal axis of the first bone screw beingsubstantially aligned with and located between the superior articularprocess of the inferior vertebra and the inferior articular process ofthe superior vertebra, the second bone screw being inserted into thesecond facet joint gap between the superior articular process of theinferior vertebra and the inferior articular process of the superiorvertebra such that an externally threaded shaft of the second bone screwpartially engages the superior articular process of the inferiorvertebra and the inferior articular process of the superior vertebra, alongitudinal axis of the second bone screw being substantially alignedwith and located between the superior articular process of the inferiorvertebra and the inferior articular process of the superior vertebra;and c) removing the first and second aiming wires from the first andsecond facet joint gaps.
 7. The method of claim 6, wherein the first andsecond bone screws are percutaneously inserted into the first and secondfacet joint gaps in step (b).
 8. The method of claim 6, furthercomprising the step of: d) placing first and second marks on a patient'sskin to identify first and second articulating planes of the first andsecond facet joint gaps; and e) forming bilateral skin incisions at thefirst and second marks.
 9. The method of claim 6 comprising the furtherstep of: d) inserting a first trocar at least partially into the firstfacet joint gap and a second trocar into the second facet joint gapprior to step (a).
 10. The method of claim 6 wherein the first andsecond facet joint gaps are prepared prior to step (a) with a drill bit.11. The method of claim 6 wherein the first and second bone screws areinserted in step (b) until heads of the first and second bone screwstouch bone adjacent the first and second facet joint gaps.
 12. Themethod of claim 11 comprising the further step of: (d) placing first andsecond anti-rotation elements over the heads of the first and secondbone screws.
 13. The method of claim 6 comprising the further step of:(d) verifying placement of the first and second bone screws with animage identifier prior to step (c).
 14. A method of inserting at leastone intra-facet bone screw and an anti-rotation element into a facetjoint gap between superior and inferior articular processes of adjacentvertebrae, the at least one bone screw including a longitudinal axis, anexternally threaded shaft, a head and a cannulated bore and theanti-rotation element including two legs and a central portion, themethod comprising the steps of: a) forming a skin incision; b) insertingan aiming wire at least partially into the facet joint gap; c)percutaneously inserting the at least one bone screw at least partiallyinto the facet joint gap using the aiming wire, the at least one bonescrew being inserted between the superior articular process of aninferior vertebra and the inferior articular process of a superiorvertebra, d) rotating the at least one bone screw into the facet jointgap such that the externally threaded shaft of the at least one bonescrew partially engages the superior articular process of the inferiorvertebra and the inferior articular process of the superior vertebra,the longitudinal axis of the at least one bone screw being substantiallyaligned with and located between the superior articular process of theinferior vertebra and the inferior articular process of the superiorvertebra and the head touches bone adjacent the facet joint gap; e)removing the aiming wire; f) placing the anti-rotation element over thehead such that the two legs are positioned in the facet joint gap,respectively, and the central portion is positioned in the cannulatedbore; and f) closing the incision.
 15. The method of claim 14, whereinthe at least one bone screw is inserted in step (c) by moving the atleast one bone screw down the aiming wire such that the aiming wire ispositioned within the cannulated bore.
 16. The method of claim 14comprising the further step of: g) turning the at least one bone screwback following step (d) to match grooves in the head of the at least onebone screw with the joint gap such that the two legs are positioned inthe facet joint gap following step (f).